The present invention relates to clutch control apparatus and method for controlling a torque transmitted between an input rotating member and an output rotating member of a power transmission system.
Conventionally, there is known a clutch control technique for automotive vehicle, which controls the magnitude of a torque transmitted from an engine to a drive wheel by automatically engaging and disengaging a clutch during vehicle starting or running. A Published Japanese Patent Application No. H9-72353 (hereinafter referred to as “JP9-72353”) shows a clutch control technique that derives a clutch torque capacity coefficient in accordance with a clutch speed ratio defined as a ratio of the rotational speed of a driven-side rotating member to the rotational speed of a driving-side rotating member, using a predetermined map defining a relationship between the clutch speed ratio and the clutch torque capacity coefficient, computes a desired torque capacity in accordance with the clutch torque capacity coefficient and an engine speed, and automatically controls the clutch torque capacity in accordance with the desired clutch torque capacity by regulating the hydraulic pressure of a clutch piston to adjust the engagement pressure of the clutch. In JP9-72353, three different maps concerning clutch the torque capacity coefficient are prepared and selected in accordance with a throttle opening. In each of the maps, the clutch torque capacity coefficient is set to a minimum value when the clutch speed ratio is identical to 1.0 indicating a fully engaged state of the clutch. Specifically, each of the maps is defined in such a manner that as the clutch speed ratio increases or decreases from 1.0, to increase the amount of slip of the clutch, the clutch torque capacity coefficient increases. For example, during vehicle starting, the clutch torque capacity is set to increase with an increase in the clutch slip speed, and the clutch engagement pressure is automatically controlled, to promote the clutch speed ratio to change toward 1.0. On the other hand, during vehicle steady-state driving where the clutch is fully engaged, the clutch torque capacity coefficient is set to be smaller, and the clutch torque capacity is computed to be small accordingly, so that the clutch engagement pressure is comparatively small. As a result, the torque transmitted through the clutch does not increase excessively large, and a potential engagement shock in engaging the clutch is reduced.
In the above-mentioned torque capacity control, the clutch torque capacity coefficient is set to be higher to some extent than the actual value of the torque transmitted from the driving side to the driven side, when the clutch speed ratio is 1.0. That is, the clutch is controlled to be in a state where the driven side and the driving side are engaged by an engagement pressure higher than an actually required engagement pressure, to ensure full torque transmission.